The long term objective of this work is to understand the mechanisms of sensory transduction in mammalian somatic receptors that sense mechanical changes. Several steps are involved in this process: transmission of the mechanical signal to the sensory terminals of the primary afferent neuron, conductance changes in the terminals-leading to the receptor potential, its electrotonic spread to the site of impulse-initiation and propagation of action potentials in the sensory axon. While the impulse train in response to a mechanical stimulus, i.e. the end result of the sensory transduction, is well known, there are very few data on the several steps leading to the sensory message sent to the C.N.S. This proposal is for study of impulse-initiation in preparations of several mechanoreceptors which can be studied in isolation. Using microscopic visualization with image processing and high speed photodetectors to record rapid changes, electrical potential changes in different regions of the sensory endings will be detected by voltage- sensitive fluorescent probes. Electrical recording with micropipettes would also be used. The site, or sites of impulse initiation, as well as their subsequent propagation, will be studied in Pacinian corpuscles and in muscle spindles. Are there multiple pacemaker sites in receptors supplied by several branches of the sensory axon? Is there propagation antidromically toward the sensory terminals? If so, how does this affect the spread of the receptor potential and the consequent sensory impulse train sent to the C.N.S? This information, in an area which has been barely explored, could greatly improve our understanding of how mechanical events are sensed. This could open up new areas of inquiry of possible importance in sensory deficits.